A. Field of the Invention
The present invention relates to a method for routing datagrams across wireless networks, especially to a method which integrates the communication mechanisms of Mobile IP (Internet Protocol) and IAPP (Inter Access Point Protocol) for routing datagrams between a MAC layer and a network layer, thereby to allow a mobile station to roam among various Access Points in different IP subnets.
B. Description of the Prior Art
The current IEEE 802.11 standard only defines the protocols below the MAC layer for wireless LAN (hereinafter WLAN). The protocols on network layer have not been defined for WLAN yet. The communication among the Access Points (AP) in the WLANs is following the communication mechanisms defined by the Inter Access Point Protocol (IAPP). Accordingly, the IAPP is implemented on top of IP and is an extension to the exist management protocols. By specifying how Access Points will communicate with one another, the IAPP defines methods to handle access point awareness as well as roaming by mobile stations across cells. According to IAPP, when an AP is enabled, it broadcasts an announce request packet in the WLAN. Another AP in the same LAN will reply with a response packet upon receiving the announce request packet. In the response packet, it contains the information about the AP, such as the MAC address and IP address of the AP. When a mobile station roams to another WLAN served by another AP, the mobile station will send a reassociation request to the new AP. The new AP will have to find the IP address of the AP originally served for the mobile station so as to complete a handover request procedure, which will allow the new AP to replace the role of the original AP and provide services for the mobile station.
However, if the mobile station moves to another WLAN in a different IP subnet, the conventional communication mechanism will be blocked by the router. Consequently, the new AP will not be able to obtain the IP address of the original AP via broadcasting. The reason is because most routers are configured to block the traffic of broadcast packets so as to prevent the occurrence of broadcast storm. For this reason, APs in different IP subnets will not be able to communicate with one another via broadcasting packets as defined by the IAPP. In another words, when a new AP receives a reassociation request from a mobile station, it has no way to get the IP address of the original AP if they are in different IP subnets. Consequently, a handover procedure will not be completed. As a result, the original AP might determine that it still serves for the mobile station. At this time, the original AP will not forward any frame that destines to the mobile station sent by another mobile station in the same WLAN. Such restriction will seriously limit the roaming area of a mobile station.
Accordingly, it is an object of the present invention to provide a method which integrates the communication mechanism of mobile IP and IAPP, thereby to allow mobile stations to roam across various Access Points in different IP subnets.
It is another object of the present invention to provide a communication mechanism which can readily allow mobile stations to roam across various Access Points in different IP subnets without the requirement of extra hardware implementation or software configurations.
In accordance with the invention, a method involving in integrating the communication mechanisms of IAPP and mobile IP is provided to allow a mobile station to roam among various APs of different IP subnets. When a mobile station roams to a new IP subnet, it will broadcast a reassociation request to an Access Point A in another IP subnet. In response to the reassociation request, the Access Point A will need the IP address of the previous Access Point B in the previous IP subnet to forward the handoff request to the Access Point B. So, the Access Point A can find the IP address of the Access Point B via the communication mechanism of mobile IP of IP layer and then send the handoff request frame to the Access Point B. In turn, upon receiving the handoff request frame, the Access Point B deletes the record of the mobile station in the association table and then sends the handoff response frame back to the Access Point A via the communication mechanism of mobile IR The unicast handoff response frame will be forwarded to the Access Point A. Consequently, the Access Point A can complete the handover procedure.